Investigating gene regulatory networks in aorti arch artery development

Abstract

Developmental defects to the heart and aortic arch arteries are a leading cause of morbidity and occur in 22q11 deletion syndrome (22q11DS) patients. Formation of the aortic arch arteries requires the remodelling of the pharyngeal arch arteries (PAA) which depends on regulated gene expression and the interaction of multiple tissues. TBX1 has been identified as a leading causative gene in 22q11DS, however, the wide spectrum of defects present in 22q11DS patients suggests that modifier genes may contribute to this phenotypic variation. Tbx1, Pax9 and Gbx2, all independently required for cardiovascular development, are co-expressed in the pharyngeal endoderm, a tissue that provides signalling cues during PAA morphogenesis. Tbx1 and Pax9 genetically interact and Gbx2 is downregulated in both Tbx1-null and Pax9-null mice. The aim of this project was to establish the Gbx2-null phenotype and investigate a potential interaction between Gbx2 and Pax9 in the pharyngeal endoderm during cardiovascular development. Gbx2-null mouse embryos presented with 4th PAA-derived defects, such as right aortic arch, as well as outflow tract defects. Pax9+/-;Gbx2+/- mice were crossed to study the interaction between these genes by generating embryos with complex genotypes. The presentation of cardiovascular defects in Pax9+/-;Gbx2+/- mice showed a strong genetic interaction. Likewise, Pax9 heterozygosity modified the Gbx2-null phenotype, both increasing the penetrance of defects and causing the presentation of additional abnormalities. Conditionally deleting Gbx2 from the endoderm concomitantly with the heterozygous deletion of Pax9 resulted in cardiovascular defects, highlighting the pharyngeal endoderm as a key tissue in PAA morphogenesis and remodelling. Mouse models were used to study a potential Tbx1-Pax9-Gbx2 genetic network in cardiovascular development. In vitro models were used to investigate the interaction of Tbx1 and Pax9 with the Gbx2 coding region. The data presented in this thesis suggests that a Tbx1-Pax9-Gbx2 genetic network exists within the pharyngeal endoderm to control PAA morphogenesis.